At the time, Canada was still a part of the British Empire, and it was due to WWI that Canada became its own dependent nation. So, this may be a starting point for your poster. Keep in mind that propaganda was a big player in all of the countries during WW I and can be defined as follows:

Since the nation was being persuaded to enter into war, you could use the statement such as "The Other Side of the Atlantic Needs You" or "Separated Doesn't Mean Betrayal" or "We'll have our British Brothers Backs" or something to persuade by emotion and national heritage. As far as illustrating, it all depends on the subject of your persuasion, but flags and soldiers would always work. Take a look at a few real historical Canadian posters from WWI to give you ideas of illustrations.

Perhaps an appeal can be made to the citizens of Canada who are Scottish, since the Scottish are known for being among the best soldiers in the world. In 1912 there was a Scottish Regiment begun in Vancouver. Here is a link that you can peruse for more information:

I think you can appeal to their sense of honor and fair play. Call upon them to make the world safe for democracy. When it comes to propaganda, you want to use powerful, emotional words that will make them feel obligated to represent their country and save the world.